The present invention relates to polypropylene films of high opacity and low light transmission, and particular to oriented films containing voids prepared from a film-forming polymer and particles of cyclic olefinic copolymers (COC) dispersed therein.
In the packaging of certain types of foods, such as snack foods like potato chips, cookies and the like, it is common practice to employ a multilayer film. A desirable property in such a packaging film is an opacity which protects the packaged product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light, up to about 450 nm cause increased spoilage in such packaged products. Even when a degree of opacity is present in the film, spoilage occurs if the film allows passage of some light. One way of increasing opacity in a multilayer film is by providing a layer or stratum of voids which hinders passage of light.
It is known to provide a layer of voids in thermoplastic films by loading cavitating agents in thermoplastic polymer which is cast into a film, and thereafter stretching to form oriented thermoplastic films. Different cavitating agents can be employed under particular processing conditions to obtain desired opaque, oriented polymeric films.
Attempts to vary the types of cavitating agents have been made to improve opacity and machinability of oriented polymeric films. For example, U.S. Pat. Nos. 4,758,462, and 5,176,954, disclose the use of organic polymers such as polybutylene terephthalates as cavitating agents in oriented polypropylene matrix materials. U.S. Pat. No. 4,758,462 to Park et al., relates to polymeric films of enhanced opacity and methods of making the same. The films of the '462 patent are made with a thermoplastic polymer matrix material within which is located a stratum of voids. The voids can be created by polybutylene terephthalate cavitating agents.
U.S. Pat. No. 5,176,954 to Keller et al., is directed to a non-symmetrically layered, highly opaque, biaxially oriented polymer film having a core layer which contains polybutylene terephthalate cavitating agents, as well as iron oxide, aluminum and titanium dioxide.
The polybutylene terephthalates described in the above patents, are good cavitating agents that can be processed at high temperatures (i.e., temperatures higher than the melting point of the matrix material). Polybutylene terephthalates, however, are sensitive to hydrolytic breakdown, and thus can degrade into lower molecular weight materials. These low molecular weight materials have been known to migrate to surfaces of processing apparatus, e.g., melt pipes, filters, dies, etc. These materials build up and can then eventually slough off the metal surfaces and pass into the films as sizable deposits of hard, eggshell-type impurities which cause the film to split.
Nylon cavitating agents, on the other hand, are not as likely to undergo hydrolytic breakdown and dispersion. However, nylon cavitating agents cannot generally be used at high temperatures. For example, U.S. Pat. No. 4,377,616 discloses that when nylons are used as cavitating agents in a polymeric matrix, the drawing temperature of the film can be quite close to the melting point of the polymeric matrix material.
Attempts have also been made to use other cavitating agents with a polymeric matrix to produce opaque, oriented films. In U.S. Pat. Nos. 5,134,173 and 5,188,777 to Joesten et al., cross-linked polystyrenes are used as cavitating agents to make opaque, biaxially oriented polymeric films.
The present invention overcomes shortcomings of the prior art and improves the manufacturing capability for making biaxially oriented polymeric films which are provided with opacity by use of cavitating agents.